Device and method for operating a refrigeration cycle with noncondensable gas addition.

ABSTRACT

The present invention relates to a device and method for operating a refrigeration cycle with the addition of noncondensable gas.

BACKGROUND OF THE INVENTION

The present invention relates to the field of the refrigeration plants, air conditioners and heat pumps operating a vapor compression cycle, in subcritical condition, where at least 90% of the molar concentration of the refrigerant fluid does condensate in the condenser, were the refrigerant has critical temperature higher than 310 kelvins and normal boiling point over 220 kelvins (so excluding most of cryogenic application); eventually with an hot gas bypass for defrost or heating purpose, or a four-way valve to operate a cycle inversion as in the heat pumps.

In particular the present invention describes a method and a device to operate a refrigeration cycle with the addition of noncondensable gas and a reduced refrigerant charge, to obtain improved or simplified operation for:

refrigerant subcooling at condenser outlet;

hot gas defrost if any;

heat pump defrost if any.

Some plain examples of the field of application are:

refrigeration plant using R404A refrigerant, for an ice cream cold room at −25° C.;

refrigeration plant using R134a refrigerant, for a meat storage cold room at 0° C.;

water chiller using R410A refrigerant, for chilling water at +7° C.;

conditioner with heat pump using R407C, for keeping a room at +23° C.

traditional refrigeration plant or heat pump using R22 refrigerant.

The vapor compression cycle is well known and described widely as for instance in U.S. Pat. No. 6,701,729 of the inventor Alan W. Bagley.

In the state of the art, the subcooling of the liquid refrigerant at the condenser exit is determined by the dynamical equilibrium of the circuit components and by the refrigerant charge. A bigger liquid subcooling leads to increased cooling capacity and increased COP, as long as the high pressure of the circuit remains the same. A bigger subcooling can be obtained installing additional components, for instance using 2-stage compressors and subcoolers/economizers, or even modifying the condenser circuit to have a subcooling section. The simplest method is however to fill more refrigerant as to flood the final part of the condenser, so acting as subcooler and not just as condenser. A bigger quantity of refrigerant is however not desirable for cost, for hazards connected to refrigerant leakage and for greenhouse effect of some refrigerants.

In the state of the art, hot gas defrosting is a simple method to perform defrost, that is gaining popularity in the recent years in the field of refrigeration plants serving rooms at temperature of 0° C. or less. The hot gas defrost consists of installing a piping from the compressor outlet to the evaporator inlet, bypassing the condenser. During hot gas defrost the compressor is on and is the energy source. A solenoid valve is installed on the hot gas piping and is open just during the hot gas defrost.

The hot gas technology replaces the electric defrost, often reducing consumption and simplifying the maintenance. In the simplest arrangement it is possible to use a standard evaporator without even bypassing the evaporator distributor. This way the hot gas flow is strongly reduced by the evaporator distributor, leading to a soft and slow defrost, albeit completely safe without risk of liquid refrigerant coming back to the compressor.

On the other side hot gas defrost suffers from seasonal variation of outdoor temperature: in the cold season the low pressure of the circuit can go under the atmospheric pressure, forcing the low pressure switch to stop the system, to avoid the risk of air entering the circuit. The simplest solution is again to fill a bigger quantity of refrigerant, as to flood the condenser, to rise the high pressure and so the low pressure as well. As said above, a bigger quantity of refrigerant is however not desirable.

In the state of the art, in an air conditioner with heat pump operating with outdoor temperature near or below 0° C., the outdoor condenser, when used as evaporator, can accumulate ice on the finned surface, so reducing the heat exchange capacity. The common solution is to invert back the cycle periodically, operating the heat pump as an air conditioner to defrost the condenser. In cold climate, around −10° C., often this method is unable to defrost the lower part of the condenser, near to the refrigerant outlet (becoming the inlet during heat pump operation). To solve this partial defrost, it is possible to install additional electric heaters in the condenser, or more simply it is enough to overfill the heat pump by a bigger refrigerant quantity, but, again, a bigger quantity of refrigerant is not desirable.

In summary, in the state of the art, a bigger quantity of refrigerant can be used to improve subcooling, hot gas defrost and heat pump defrost. This invention discloses a simple method and a simple device to get similar results with a reduced quantity of refrigerant.

BRIEF SUMMARY OF THE INVENTION

The purpose of the invention is to operate a refrigeration plant especially suitable for the cold season, with reduced refrigerant charge, bigger subcooling at condenser outlet, improved hot gas defrost and improved heat pump defrost.

Without this invention, a special arrangement or an increase of the quantity of refrigerant would be otherwise necessary, during defrost, for compensating the cold season and, during refrigeration, for getting bigger subcooling at condenser outlet, if required.

This invention discloses a method and a device with addition of noncondensable gas, a suitable controller and suitable metering means, to reduce the quantity of refrigerant and to get improved behavior during the cold season.

The addition of non condensable gas increases the subcooling and improves the defrost.

A suitable controller and suitable metering means make the device more tolerant to bigger quantity of vapor at the condenser outlet, so contributing to reduce the refrigerant charge and adapting to the seasonal variations.

The effect of a small gas addition is amplified by the circuit as to get a propeller effect in the hot gas defrost and an indirect reduction of the liquid refrigerant temperature at the condenser outlet. The negative effects of the gas addition are tamed, as better explained in the following.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

These and other characteristics of the invention will be clear from the following description of a preferential form of embodiment, given as a non-restrictive example, with reference to the attached drawings wherein:

FIG. 1 shows a refrigerating circuit with a bypass for hot gas defrost;

FIG. 2 shows a refrigerating circuit with cycle inversion for heat pump operation.

DEFINITIONS AND FACTS

As used herein, “molecular oxygen” refers to O₂, the diatomic oxygen molecule. It is well known that diatomic oxygen gas constitutes about 21% of the volume of air, has a normal boiling point of about 90 kelvins and critical temperature of about 155 kelvins.

As used herein, “molecular nitrogen” refers to N₂, the diatomic nitrogen molecule. It is well known that diatomic nitrogen gas constitutes about 78% of the volume of air, has a normal boiling point of about 77 kelvins and critical temperature of about 126 kelvins.

As used herein, “atomic helium” refers to He, the monatomic helium gas. It is well known that atomic helium gas constitutes about 0.0005% of the volume of air, has a normal boiling point of about 4 kelvins and critical temperature of about 5 kelvins.

As used herein, “carbon dioxide” refers to CO₂, the molecule composed by one atom of carbon and two atoms of oxygen. It is well known that carbon dioxide gas constitutes about 0.04% of the volume of air, has a normal boiling point of about 217 kelvins and critical temperature of about 304 kelvins.

As used herein carbon dioxide is not a “pure refrigerant fluid”.

As used herein carbon dioxide is a “pure noncondensable gas”.

As used herein, “pure fluid” refers to a fluid composed by just one chemical substance, as opposed to a mixture. So pure water is a pure fluid being composed just of H₂O, molecular nitrogen also is a pure fluid being composed just of N₂, while air is not a pure fluid being composed of molecular nitrogen, molecular oxygen and several other gases.

As used herein, “pure gas” refers to a pure fluid (as opposed to a mixture) being also a gas.

As used herein, “critical temperature”, referred to a pure fluid, refers to the temperature at and above which vapor of the fluid cannot be liquefied, no matter how much pressure is applied. This is more precisely called the temperature of the vapor-liquid critical point.

As used herein, “normal boiling point”, referred to a pure fluid, refers to the temperature at which the vapor pressure of the liquid equals the atmospheric pressure.

As used herein, “pure refrigerant fluid” refers to a pure fluid (as opposed to a mixture) having critical temperature higher than 310 kelvins and normal boiling point over 220 kelvins, used for heat transfer in a refrigerating system, which absorbs heat at a low temperature and a low pressure and rejects heat at a higher temperature and a higher pressure usually involving changes of the state of the fluid. As used herein carbon dioxide is not a “pure refrigerant fluid”.

As used herein, “refrigerant fluid” refers to a fluid (pure or mixture) constituted for at least 90% of its molar concentration by pure fluid refrigerants. In particular, as used herein, carbon dioxide molar concentration can not exceed 10% of a refrigerant.

As used herein, “pure noncondensable gas” refers to a pure gas having critical temperature not higher than 310 kelvins and normal boiling point not over 220 kelvins, added to or contained in or mixed with or adulterating at least one phase of a refrigerant fluid, by the fact of being contained in the same circuit, and persisting in the gaseous state under the prevailing conditions. As used herein carbon dioxide is a “pure noncondensable gas”.

As used herein, “noncondensable gas” refers to a pure noncondensable gas or to a mixture of pure noncondensable gases. Noncondensable gas can be contained in a refrigerant or even mixed with it due to an addition in the circuit of the refrigeration plant. Non condensable gas that does not mix with the refrigerant liquid phase does however mix with the vapor phase of the refrigerant.

As used herein, “molar concentration of a pure noncondensable gas”, when compared to a refrigerant fluid, refers to the number of moles of the specified pure noncondensable gas divided by the number of moles of the refrigerant (in both vapor and liquid phases). This concentration is an average in the circuit. This concentration can be expressed in percentage (%) as usual, where for instance a molar concentration of 1% of the refrigerant refers to a ratio of 0.01.

As used herein, “average value of the overall molar concentration of the noncondensable gases”, when compared to a refrigerant fluid, refers to the overall number of moles of all of the pure noncondensable gases divided by the number of moles of the refrigerant (in both vapor and liquid phases). This concentration is an average over the circuit. This concentration can be expressed in percentage (%) as usual, where for instance a molar concentration of 1% of the refrigerant refers to a ratio of 0.01.

As used herein, “refrigerant overheating” refers to the amount by which the temperature of a superheated vapor exceeds the dew temperature of the vapor at the same pressure.

As used herein, with the exception of the refrigerant definitions said above, all the definitions of U.S. Pat. No. 6,701,729 of the inventor Alan W. Bagley are taken verbatim, as contained in the paragraph named “DEFINITIONS” of U.S. Pat. No. 6,701,729. For sake of completeness the most relevant definition is repeated hereunder.

As used herein, a “metering means” or “metering device” both refer to any device that is capable of receiving a liquid at a first pressure at its inlet and expelling that liquid at a second, reduced pressure at its outlet. Such devices include, without limitation, a simple orifice, an orifices [orifice] containing a floating piston, a flow restrictor, a capillary tube and a thermostatic expansion valve (TXV). These and other such devices are well-known in the art and all are within the scope of this invention.

DETAILED DESCRIPTION OF THE INVENTION

The interaction of noncondensable gas with refrigerant has been described in particular in 1993 by J. C. Yang, B. D. Breuel, and W. L. Grosshandler of Building & Fire Research Laboratory—National Institute of Standards & Technology—Gaithersburg, Md. 20899 (see http://www.fire.nist.gov/bfrlpubs//fire93/art127.html).

The authors investigated in particular the solubility of molecular nitrogen in several refrigerants, as described by the so called Henry's constant, in a study to expedite the discharge of the refrigerant contained in the fire extinguishers, thanks to the nitrogen addition. An eight percent mass of nitrogen compared to the mass of refrigerant, has been reported as sufficient to improve the efficiency of a fire extinguisher, in a similar way as it is used in this invention to improve the hot gas defrost; in this invention however the mass of required gas is much smaller thanks to the amplification effect described below.

The Henry's constant of nitrogen in R134a has been reported to be around 66 MPa, while nitrogen penetration in R22 has been reported as practically negligible.

The Henry's law states that at a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. This is valid for low concentrations.

So the gas partial pressure is proportional to the molar concentration of the gas in the liquid, the constant of proportionality being here named Henry's constant and measured in MPa.

As a preferential embodiment, FIG. 1 depicts a device comprising a compressor (1), a condenser (9), a piping (6) from the compressor outlet (3) to the condenser inlet (10), a solenoid valve (12) used as metering means, a piping (15) from condenser outlet (11) to the valve inlet (13), an evaporator (17), a piping (16) from the valve outlet (14) to the evaporator inlet (18), a piping (20) from the evaporator outlet (19) to the compressor inlet (2), a hot gas piping (7) from the compressor outlet to the evaporator inlet and a hot gas solenoid valve (8) on this piping.

A refrigerant circulates from the compressor to the condenser, to the metering solenoid, to the evaporator and back to the compressor in a refrigeration cycle.

A controller (21) does receive a signal from a pressure probe (4) and from a temperature probe (5), wherein both probes are positioned between the inlet of the evaporator and the outlet of the compressor, preferably near to the compressor inlet, as to regulate the refrigerant feeding of the evaporator, by driving of the metering solenoid.

The present invention is characterized by the addition of noncondensable gas, mixed with the refrigerant or added to the circuit in any way.

This circuit, known in the state of the art, is described for instance in EPO patent nr. EP1607699 of the same inventor of present invention. The metering solenoid, not having any orifice, is more tolerant to vapor refrigerant at condenser outlet, so further reducing the quantity of refrigerant. The solenoid valve is pulsed on and off as to get the required overheating at the condenser outlet.

Instead of the metering solenoid, any other metering means could get, at least partially, the benefits of present invention. To mention just a few of them: a traditional mechanical thermostatic valve like Danfoss TEX 2, an electronic thermostatic valve with orifice like Danfoss AKV 10, a capillary are all perfectly able to operate with the addition of noncondensable gas, but they need a bigger charge of refrigerant.

The metering solenoid without the orifice, as driven by the controller as described here and in the latter patent, is simply the preferred metering device, because the inventor has experimentally verified it can reduce the required refrigerant quantity.

The invention is further characterized by the choice of refrigerant, the molar concentration of non condensable gas and the lack of oxygen.

The refrigerant of present invention, for at least 90% of the molar concentration, consists of one or more pure fluids having critical temperatures higher than 310 kelvins and normal boiling point over 220 kelvins. This is to limit the invention to the range in which it is believed useful and to distinguish the invention from the field of cryogenic, where different devices can employs non condensable gas for different reasons and with different methods.

In the present invention, the noncondensable gas addition has an overall molar concentration comprised between 0.4% and 1% of the refrigerant fluid. The noncondensable gas has critical temperature not higher than 310 kelvins and normal boiling point not over 220 kelvins. This is again to limit the invention to the range in which it is believed useful and to distinguish it from accidental similarities with other unrelated inventions.

In the present invention, the molar concentration of the eventual molecular oxygen in the noncondensable gas addition is limited to 0.02% of the refrigerant. This is because oxygen presence inside a refrigerating circuit is in itself undesirable for the chemical stability of all the materials inside the circuit, but also to avoid covering the accidental presence of air inside a refrigerant circuit caused by improper or unlucky operation of a refrigeration plant.

The invention is further characterized by being in the subcritical region, having condensation of at least 90% of the refrigerant mass in the condenser.

All of the aforementioned restrictions aim to circumscribe the gas addition to a plain refrigeration circuit, where the gas is used just as a propeller for hot gas defrost and as an indirect method of reducing the liquid partial pressure and so increasing the subcooling, as better explained below.

In this invention the noncondensable gas is not segregated, nor separated, nor collected in any dedicated part of the circuit. The gas circulates in the refrigeration cycle with both the liquid and vapor phases of the refrigerant.

Charging the circuit of FIG. 1 by the R22 refrigerant and putting a molecular nitrogen addition, as to have a nitrogen to refrigerant molar concentration of 0.5%, the mechanism of subcooling and improved defrost can be easily attained and explained.

During hot gas defrost, most (typically around 95%) of the mass of the R22 refrigerant is inside the condenser in liquid phase, while the nitrogen, not mixing with liquid refrigerant, is mixed just with vapor refrigerant. So the molar concentration of nitrogen in the vapor refrigerant is amplified from the original 0.5% to around 10%, simply due to the fact that nitrogen is not present in the liquid refrigerant.

During hot gas defrost, the compressor is on, the hot gas solenoid is open and the refrigerant does bypass the condenser going straight to the evaporator.

The mass of refrigerant circulating is modest and in vapor phase, and is reasonably well described by the perfect gas law. In this context a 10% molar concentration of nitrogen contributes to about 10% of the defrost effect, by the perfect gas law.

A typical effect, as experimentally verified, is 0.9 bar rise on the low pressure side at the end of the defrost cycle.

This makes a dramatic difference in cold climate, when the low pressure side of the circuit would go otherwise under atmospheric pressure, forcing to stop the compressor to avoid air and humidity intake from eventual holes in the low pressure side of the circuit. A few tenths of a bar make the difference.

The same result (0.9 bars rise on the low pressure side) could be attained simply adding refrigerant, instead of noncondensable gas, but the typical addition, as experimentally verified, would be about 100% of the minimal charge, because the additional refrigerant would go to flood the condenser, so giving just an indirect contribution to hot gas defrost. Eventually this refrigerant addition should be removed in the hot season.

During the normal refrigeration cycle (not during defrost), the gas addition causes subcooling at the condenser outlet. At the condenser inlet, both fluids (R22 and nitrogen) are hot and can be described as perfect gases, where the partial pressure of nitrogen is about 0.5% of the partial pressure of R22. The “apparent condensing pressure” is the total pressure at condenser inlet given by both R22 and nitrogen.

As a first approximation, for small pressure drop through the condenser, the total pressure at condenser outlet is about the same as inlet.

At condenser outlet the liquid R22 and the nitrogen occupy a smaller volume than at the inlet, so the partial pressure of nitrogen does rise, while the R22 partial pressure is reduced of the same amount, as to maintain same total pressure between condenser inlet and outlet. So the R22 refrigerant at condenser outlet has a lower partial pressure and is colder than would be otherwise the same pure refrigerant (without nitrogen addition), at the same “apparent condensing pressure” and at saturation. Technically speaking the liquid refrigerant subcooling at the condenser outlet is just “apparent”, because the refrigerant is colder just because saturated at a lower partial pressure than the apparent condensing pressure; but, for cooling purpose, the apparent subcooling has the same effect of real subcooling, because the refrigerant enthalpy at condenser outlet is however lower.

A typical effect, as experimentally verified, is about 10 K (kelvins) of apparent subcooling at the condenser outlet.

To maintain the same COP, a bigger refrigerant quantity is required, to avoid excessive vapor quality at condenser exit, the typical R22 addition, as experimentally verified, is from 10% to 40% of the minimal charge, varying with the evaporation temperature, because the refrigerant subcooling has a bigger impact on COP at lower evaporating temperatures. The metering solenoid, not having any orifice, can handle the vapor quality better than a traditional device with calibrated orifice.

The effect of the gas addition is amplified at condenser outlet thanks to the fact that the liquid refrigerant occupies a much smaller volume than a perfect gas, so a small quantity of perfect gas does reduce the partial pressure of the refrigerant for an amount equivalent to 10 K of saturation temperature.

FIG. 2 depicts a variant of FIG. 1, where a four-way inversion valve (22), like the Danfoss STF model, is connected by the piping to the compressor inlet, the compressor outlet, the condenser inlet and the evaporator outlet. Where all the other components perform the same function as in FIG. 1, bearing the same numbering, with the exception of the hot gas means, not present in FIG. 2. The four-way valve is used to invert the cycle during the heat pump operation, while the metering solenoids performs its metering action even when the four-way valve does invert the cycle. The position of the metering solenoid valve and of the probes allows to regulate the refrigerant flow in both direction, using a bi-flow solenoid valve as the Danfoss EVRC.

The refrigerant fluid flows through the metering solenoid, when the four-way valve does invert the cycle, in the opposite direction of the flow of the refrigerant that occurs in the refrigeration cycle.

During the defrost of the heat pump, the normal refrigeration cycle is reinstated as to heat the condenser. The apparent subcooling at condenser outlet forces the condenser to perform refrigerant condensation up to the condenser outlet. This way there is an advantage over simply flooding the condenser by liquid refrigerant, because it is known that the heat exchange in the condensing section of the condenser is about three time the heat exchange in the subcooling section (see for instance 2004 ASHRAE Handbook—chapter 35—FIG. 6).

The condenser critical region for defrost purpose is the final part, near to the refrigerant outlet. The increased heat exchange in this region leads to better defrost.

The R22 refrigerant is being gradually phased out, but the method and the device described can be used with R22 on existing plants. The simplest method is to modify an existing and traditional R22 refrigeration plant or heat pump, by just adding molecular nitrogen in the molar concentration of 0.4% to 1% of the refrigerant, at the same time increasing the refrigerant quantity, if necessary, by 10% to 50% of the minimum charge, as required by the existing metering means to cope with the vapor refrigerant in the liquid line, without any other modification to the existing plant.

The refrigerant charge is determined by test as the minimum amount able to reduce the vapor refrigerant at condenser outlet up to the point where the passage through the metering means is sufficient to operate the machine at the required overheating at the evaporator outlet.

The atomic helium, being lighter, can be used instead of the molecular nitrogen to facilitate the passage into the orifices, the evaporator distributor and the valves.

All the legally allowed refrigerants, like R134a, R404A, R407C, R410A, R717 and so on can be used in new refrigeration plants and heat pumps, as described above. The dilution of the perfect gas inside the refrigerant, as described by the Henry's constant, being not negligible, does reduce just partially the gas amplification effects above described; the preferred embodiments of FIGS. 1 and 2 are maintained.

For devices not having any form of defrost, as for instance the water chiller, the gas addition, the controller and the metering means can be used in the same way just to increase the subcooling at the condenser outlet, in a controllable and predictable way. In this latter application the hot gas means of the FIG. 1 are simply not installed.

The plain refrigerant subcooling at the condenser outlet, even without any defrost, is useful in itself because it leads to higher cooling capacity, smaller liquid piping and, eventually, higher COP especially for application with refrigerant evaporation temperature as low as −30° C.

The plain refrigerant subcooling can be obtained by noncondensable gas addition, even without special metering means, at the cost of bigger refrigerant quantity. Anyway the refrigerant charge is experimentally verified lower than the charge required without gas addition to get the same subcooling.

The effect of noncondensable gas in this invention is linked to the molar concentration (essentially the number of molecules) and to the solubility in the refrigerant, so a wide choice of gases is available, eventually mixing several gases is possible, preferring nitrogen and carbon dioxide for the cost and availability, and helium for the small weight. 

1. A device comprising a compressor (1) comprising an inlet (2) and an outlet (3); a condenser (9), comprising an inlet (10) and an outlet (11), wherein the condenser inlet is operatively coupled to the outlet of the compressor; a metering means (12), comprising an inlet (13) and an outlet (14), wherein the inlet of the metering means is operatively coupled to the outlet of the condenser; an evaporator (17), comprising an inlet (18), an outlet (19) and an evaporative surface, wherein the evaporator inlet is operatively coupled to the outlet of the metering means and the outlet of the evaporator is operatively coupled to the inlet of the compressor; a refrigerant fluid that circulates from said compressor to said condenser to said metering means to said evaporator and back to said compressor in a refrigeration cycle; wherein at least 90% of the molar concentration of said refrigerant fluid is condensed inside said condenser during the refrigeration cycle; wherein at least 90% of the molar concentration of said refrigerant fluid consists of one or more pure fluids having critical temperatures higher than 310 kelvins and normal boiling point over 220 kelvins; characterized in that said device comprises an addition of at least one pure non condensable gas; wherein said noncondensable gas is mixed with or contained in at least one phase of said refrigerant fluid; wherein the noncondensable gases mixed with or contained in said refrigerant fluid have the average value of the overall molar concentration comprised between 0.4% and 1% of said refrigerant fluid, wherein said average concentration takes into account both the liquid and vapor phases of said refrigerant; wherein the molar concentration of molecular oxygen mixed with or contained in said refrigerant fluid does not exceed 0.02% of said refrigerant fluid; wherein said noncondensable gases do circulate in the refrigeration cycle along with both the refrigerant liquid phase and refrigerant vapor phase, not being said gases confined nor segregated nor collected in a dedicated part of the device.
 2. Device as in claim 1, wherein said noncondensable gases—in their own molar composition—are composed for at least 40% by molecular nitrogen.
 3. Device as in claim 1, wherein molecular nitrogen mixed with or contained in at least one phase of said refrigerant fluid has a molar concentration of at least 0.16% of said refrigerant fluid, wherein said concentration is an average taking into account both the liquid and vapor phases of said refrigerant.
 4. Device as in claim 1, wherein said noncondensable gases—in their own molar composition—are composed for at least 40% by atomic helium.
 5. Device as in claim 1, wherein atomic helium mixed with or contained in at least one phase of said refrigerant fluid has a molar concentration of at least 0.16% of said refrigerant fluid, wherein said concentration is an average taking into account both the liquid and vapor phases of said refrigerant.
 6. Device as in claim 1, where said metering means comprises a solenoid valve having solenoid actuator that, when is activated (energized), moves a valve member in its open position while when said actuator is de-activated (de-energized) it allows the valve member to return in its closed position.
 7. Device as in previous claim, wherein said solenoid valve is operatively coupled to a suitable controller (21) and said controller does anticipate immediately the opening of said solenoid valve in case at the same time the latter is in closed configuration and the refrigerant overheating is increased over a preset value of “maximum overheating”, wherein said overheating is measured in a position comprised between the evaporator inlet and the compressor outlet.
 8. Device as in claim 6, wherein said solenoid actuator is periodically activated/de-activated by a suitable controller (21) in order to regulate the refrigerant flow as a function of the desired refrigerant overheating, wherein said overheating is measured in a position comprised between the evaporator inlet and the compressor outlet.
 9. Refrigeration plant according to the preceding claim, characterized by the fact that said electronic controller receives a signal from a pressure probe (4) and from a temperature probe (5), wherein both probes are positioned between the inlet of the evaporator and the outlet of the compressor.
 10. Refrigeration plant according to claim 6, characterized by the fact that said metering means do not comprise any metering orifice.
 11. Device as in claim 1, characterized by the fact that said device comprises a hot gas bypass means (8), comprising an inlet, an outlet, an open position and a closed position; wherein the hot gas bypass means is operatively coupled to a controller (21) in order to carry out the defrosting and the heating of the evaporator; wherein the hot gas bypass means inlet is operatively coupled to the outlet of the compressor and the hot gas bypass means outlet is operatively coupled to the inlet of the evaporator or to an inlet of a manifold, wherein: the manifold comprises an inlet and a plurality of outlets, each outlet being operatively coupled to a different one of a plurality of inlets at different locations on the evaporative surface.
 12. Refrigeration plant according to the preceding claim, characterized by the fact that the hot gas bypass means comprises a solenoid valve and the latter is maintained closed (off) during the refrigeration cycle.
 13. Refrigeration plant according to the preceding claim, characterized by the fact that, to carry out the defrosting and the heating of the evaporator, the metering means is maintained closed, the hot gas valve is maintained open, the compressor is maintained on, thereby supplying hot gas to the evaporator without bypassing the evaporator inlet.
 14. Refrigeration plant according to the preceding claim characterized by the fact that, in order to prevent the liquid from returning to the inlet of the compressor, during the defrosting and the heating said controller monitors the value of the refrigerant overheating and, when such overheating goes down, below of a preset value, said controller closes the hot gas valve; wherein said overheating is measured in a position comprised between the evaporator inlet and the compressor outlet.
 15. Device as in claim 1, wherein a four-way inversion valve (22) is operatively coupled to the compressor inlet, the compressor outlet, the condenser inlet and the evaporator outlet; wherein said metering means performs the metering action even when the four-way valve does invert the cycle said in claim 1; wherein the refrigerant fluid flows through the metering means, when the four-way valve does invert the cycle, in the opposite direction of the flow of the refrigerant that occurs in the direct cycle said in claim
 1. 16. A method for performing a refrigeration cycle, comprising: providing a compressor (1) comprising an inlet (2) and an outlet (3); providing a condenser (9), comprising an inlet (10) and an outlet (11), wherein the condenser inlet is operatively coupled to the outlet of the compressor; providing a metering means (12), comprising an inlet (13) and an outlet (14), wherein the inlet of the metering means is operatively coupled to the outlet of the condenser; providing an evaporator (17), comprising an inlet (18), an outlet (19) and an evaporative surface, wherein the evaporator inlet is operatively coupled to the outlet of the metering means and the outlet of the evaporator is operatively coupled to the inlet of the compressor; providing a refrigerant fluid that circulates from said compressor to said condenser to said metering means to said evaporator and back to said compressor in a refrigeration cycle; wherein at least 90% of the molar concentration of said refrigerant fluid is condensed inside said condenser during the refrigeration cycle; wherein at least 90% of the molar concentration of said refrigerant fluid is composed by pure fluids having critical temperatures higher than 310 kelvins and normal boiling point over 220 kelvins; providing an addition of at least one pure non condensable gas; wherein said noncondensable gas is mixed with or contained in at least one phase of said refrigerant fluid; wherein the noncondensable gases mixed with or contained in said refrigerant fluid have overall molar concentration comprised between 0.4% and 1% of said refrigerant fluid, wherein said average concentration takes into account both the liquid and vapor phases of said refrigerant; wherein the molar concentration of molecular oxygen mixed with or contained in said refrigerant fluid does not exceed 0.02% of said refrigerant fluid; wherein said noncondensable gases do circulate in the refrigeration cycle along with both the refrigerant liquid phase and refrigerant vapor phase, not being said gases confined nor segregated nor collected in a dedicated part of the device.
 17. Method as in claim 16, wherein the refrigerant charge is determined as to get a preset value of the refrigerant overheating as measured in a position comprised between the evaporator inlet and the compressor outlet.
 18. Method as in claim 16, wherein molecular nitrogen mixed with or contained in at least one phase of said refrigerant fluid has a molar concentration of at least 0.16% of said refrigerant fluid, wherein said concentration is an average taking into account both the liquid and vapor phases of said refrigerant.
 19. Method as in claim 16, wherein atomic helium mixed with or contained in at least one phase of said refrigerant fluid has a molar concentration of at least 0.16% of said refrigerant fluid, wherein said concentration is an average taking into account both the liquid and vapor phases of said refrigerant.
 20. Method as in claim 16, providing a hot gas bypass means (8), comprising an inlet, an outlet, an open position and a closed position, wherein the hot gas bypass means inlet is operatively coupled to the outlet of the compressor and the hot gas bypass means outlet is operatively coupled to the inlet of the evaporator or to an inlet of a manifold, wherein: the manifold comprises an inlet and a plurality of outlets, each outlet being operatively coupled to a different one of a plurality of inlets at different locations on the evaporative surface.
 21. Method as in claim 16, wherein said metering means comprises a solenoid valve having solenoid actuator that, when is activated (energized), moves a valve member in its open position while when said actuator is de-activated (de-energized) it allows the valve member to return in its closed position; wherein said solenoid valve is operatively coupled to a suitable controller (21) and said controller does anticipate immediately the opening of said solenoid valve in case at the same time the latter is in closed configuration and the refrigerant overheating is increased over a preset value of “maximum overheating”; wherein said overheating is measured in a position comprised between the evaporator inlet and the compressor outlet. 